The Collaborative International Dictionary of English v.0.48:

Captor \Cap"tor\, n. [L., a cather (of animals), fr. caper to
   take.]
   One who captures any person or thing, as a prisoner or a
   prize.
   [1913 Webster]

WordNet (r) 3.0 (2006):

captor
    n 1: a person who captures and holds people or animals [syn:
         captor, capturer] [ant: liberator]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CAPTOR, war. One who has taken property from an enemy; this term is also
employed to designate one who has taken an enemy.
     2. Formerly, goods taken in war were adjudged to belong to the captor;
they are now considered to vest primarily, in the state or sovereign, and
belong to the individual captors only to the extent that the municipal laws
provide.
     3. Captors are responsible to the owners of the property for all losses
and damages, when the capture is tortious and without reasonable cause in
the exercise of belligerent rights. But if the capture is originally
justifiable, the captors will not be responsible, unless by subsequent
misconduct they become trespassers ab initio. i Rob. R. 93, 96. See 2 Gall.
374; 1 Gall. 274; 1 Pet. Adm. Dee. 116; 1 Mason, R. 14.